1) Field of the Invention
The present invention relates to a wavelength division multiplexing optical repeating transmission method and a repeating apparatus suitable for use with an optical communication system.
2) Description of the Related Art
In recent years, due to drastic increase of the use of communication networks, a demand for further increase of the network capacity is increasing. Also in optical communication systems, it is expected that the capacity, speed and distance increase more than those of systems available at present in order to satisfy the demand for increase of the network capacity.
In the present situation, practical use of a wavelength division multiplexing (WDM) optical transmission system based on the transmission speed per one channel of 10 Gbit/s (giga-bit/second) has been started. In the future, a very high-speed transmission system having a transmission speed of 40 Gbit/s or more per one channel is demanded from the point of view of the demand for further increase of the capacity, the frequency utilization efficiency and the cost.
Particularly in recent years, it is demanded to multi-functionalize an optical network system. Thus, not only a point-to-point transmission function but also functions for freely changing over the path of an optical signal such as an optical add/drop multiplexing (OADM) function and an optical cross-connect (OXC) function are demanded.
Incidentally, since the refractive index of an optical fiber varies depending upon the wavelength, even if the same fiber is used, the propagation time (speed) of light differs among difference wavelengths. This phenomenon is called chromatic dispersion. In the very high-speed transmission system, in order to cope with the multi-functionalization of a network system together with such increase of the capacity described above, it is necessary to optimally compensate for the dispersion of a transmission line to optimally compensate for waveform degradation with a high degree of accuracy.
It is to be noted that the following documents disclose techniques relating to the invention of the present application.
Patent document 1:
Japanese Patent Laid-Open No. 2000-68931
Patent document 2:
U.S. Pat. No. 6,229,935
Incidentally, in an optical transmission system having the transmission speed of 40 Gbit/s or more which is demanded for the very high-speed transmission system described above, the chromatic dispersion tolerance is very small. For example, where a wavelength division multiplexed optical signal is transmitted using the NRZ (Non Return to Zero) method of the transmission speed of 40 Gbit/s, the chromatic dispersion tolerance is less than approximately 70 ps/nm (picosecond/nanometer) which is 1/16 that of a 10 Gbit/s system.
Meanwhile, the chromatic dispersion of a transmission line fiber involves such factors of variation as the difference of the length of the transmission line, the dispersion in manufacture of the chromatic dispersion coefficient and the dispersion slope coefficient of the transmission line fiber and the dispersion compensating fiber (DCF), and the temperature variation of the fiber zero-dispersion wavelength. In order to achieve long distance transmission by a very high-speed wavelength division multiplexing transmission system, it is a significant subject to make it possible to absorb the dispersion variation and suppress degradation of a reception waveform in a reception side terminal apparatus while dispersion compensation is performed with a high degree of accuracy at an add/drop point on an optical transmission line.